The present invention relates generally to a wiper control system which controls wiper operation in accordance with rain conditions, and in particular controls intermittent wiper operation by adjusting the intervals between wiper strokes in accordance with rain conditions. More specifically, the invention relates to a wiper control circuit which can satisfactorily and successfully prevent unpredictable operation of the wiper.
In recent years, various kinds of rain condition-dependent wiper control systems have been developed and proposed. Rain sensors which produce rain condition-indicative signals having values representative of rain conditions have been developed. Control systems generally include a controller responsive to the rain indicative signal to produce a control signal which dictates wiper operations such as the intervals between intermittent wiper strokes.
For example, Published Japanese Utility Model Application No. 57-130747, published on Aug. 14, 1982, discloses an automatic control system for an automotive wiper. The automatic control system includes a vibration pick-up which detects vibrations of the front windshield due to the impact of rain drops. The output signal of the vibration pick-up is amplified, filtered and then compared with a threshold to produce a driver signal when the output signal level exceeds the threshold level. In addition, Published Japanese Utility Model Application No. 57-181655, published on Nov. 17, 1982, discloses a wiper control system which is provided with a rain sensor and a comparator for comparing the rain sensor signal, the value of which gradually increases at a rate depending upon rain conditions. The comparator produces a drive signal when the sensor signal value reaches a predetermined threshold to intermittently operate the wiper.
In such a wiper control system, the rain sensor signal value is effectively integrated until the integrated value reaches the predetermined threshold. Generally, the integrated value is cleared in response to output of the drive signal and integration of the value starts immediately after clearing the preceding integrated value. Therefore, under relatively heavy rain conditions, it is possible that the integrated value from the subsequent integration operation will exceed the threshold before the wiper completes the preceding cycle of operation. This may result in irregular and unpredictable wiper operation.
On the other hand, significant changes in wiper operation interval on a cycle-to-cycle basis may disturb the driver with an irregular rhythm. This irregular wiper operation may result in conventional wiper control systems.